Temperature responsive control means for the magnetic field of a cyclotron



Aprll 18, 1967 K. G. STANDING 3,315,194

TEMPERATURE RESPONSIVE CONTROL MEANS FOR THE MAGNETIC FIELD OF A CYCLOTRON Filed Aug. 26, 1965 3 Sheets-Sheet 1 I I l f/w 774 Inn a0 & uooc 200C 500 5 Jam 42.44, i432? firronws VJ April 18, 1967 K. G. STANDING 3,315,194

I TEMPERATURE RESPONSIVE' CONTROL MEANS FOR THE 7 MAGNETIC FIELD OF A CYCLOTRON Filed Aug. 26, 1963 3 Sheets-Sheet 2 K- GAUSS April 18, 1967 K. G. STANDING 3,315,194

TEMPERATURE RESPONSIVE CONTROL MEANS FOR THE MAGNETIC FIELD OF A CYCLOTRON Filed Aug. 26, 1963 3 Sheets-Sheet 5 'tioned between the poles of an electromagnet.

United States Patent l 3 315,194 TEMPERATURE RESP ONSIVE CONTROL MEANS FOR THE MAGNETIC FIELD OF A CYCLOTRON Kenneth G. Standing, Winnipeg, Manitoba, Canada, as-

signor to Canadian Patents and Development Limited,

Ottawa, Ontario, Canada, a company Filed Aug. 26, 1963, Ser. No. 304,550 8 Claims. (Cl. 335-210) This invention relates to improvements in cyclotrons and like particle accelerators.

In one specific application, the invention is concerned with improvements in a cyclotron of the spiral ridge type. This type of machine (also sometimes called a sectorfocused, isochronous or azimuthally varying field cyclotron) differs from the basic cyclotron in its method of vertical focussing, that is to say its method of keeping the accelerated particles confined to the neighorhood of a chosen horizontal plane during their many orbits.

In all cyclotrons, protons or other charged particles are liberated in the center of a vacuum chamber posi- A pair of open-sided, hollow electrodes, known as Ds, are energized to provide an alternating electric field from which the charged .particles gain energy. Movement of the particles in a plane perpendicular to the magnetic field forces them to spiral outwardly from the center, their rate of energy gain depending on the accelerating voltage applied to the Ds. Protons will normally make at least a few hundred revolutions before reaching their full energy at the edge of the magnet, and almost all of them would 'strike one of the Us if they were free to move in the with radius. Such a field bulges radially outwardly and exerts a force on any particle straying vertically, tending to push the particle back towards a central plane. A magnetic field decrease of a few percent from the center to the edge of the magnet is sufficient to provide adequate focussing, but there are disadvantages in this method. In

a fixed frequency cyclotron the acccelerating voltage on the Ds oscillates at a constant frequency equal to the characteristic frequency with which the particles to be accelerated (for example protons) rotate about the center.

The'voltage will then theoretically have the correct phase to accelerate 'a proton each time it crosses the gap between the Ds. But When the magnetic field decreases with radius, protons will take longer to make a revolution about the center when rotating with a larger radius and this effect is added to by the relativity increase of particle mass with energy. Protons cross the D gap successively later as they move outwardly, until at some point they begin to be decelerated. This problem has limited existing fixed frequency proton cyclotrons to energies of 25 rnev., and it has even been difficult to reach this energy.

Frequency modulation of the accelerating voltage on the Ds can be used to overcome this limitation. Each time the frequency passes through the correct value for acceleration at the center, protons there will start to gain energy, and, if the frequency of the electric field is decreased in synchronism with the movement of these protons to larger radii, they will continue to gain energy and their maximum energy is then only limited by the size of the magnet. However, the protons arrive at the outside of a frequency modulated cyclotron during only a small 3,315,194 Patented Apr. 18,. 1967 Ice part of each modulation cycle (approximately 1%) so that the resulting beam is sharply pulsed. As a result, the total beam current is reduced by a factor of about a hundred below that yielded by a fixed frequency cyclotron. Also the pulsing effect itself is a disadvantage in many experiments where a constant beam is required.

A ridge machine combines the advantages of both varieties of cyclotron, namely the large beam current of a fixed frequency cyclotron with the achievement of energies that were previously only obtainable with frequency modulated machines. This result is achieved by increasing the magnetic field from the center of the cyclotron radially outwardly. The D voltage oscillates at a fixed frequency (for example 28.5 megacycles per second) so that the protons always cross the D gap in phase, and the average magnetic field is increased radially in relation to the relativity increase in the mass (for example from 18.7 to 19.8 kilogauss). In this way a large beam current (for example greater than a hundred microamps or 6 10 protons per second) is typically obtainable without the energy limitation of a normal fixed frequency machine.

However, the increasing of the magnetic field from the center outwardly aggravates the problem of vertical focussing, since the magnetic lines of force now tend to bulge radially inwardly and to create forces that push a vertically wandering proton even further away from the median plane. It is thus necessary to provide a focussing force large enough to overcome this defocussing effect, as well as to provide enough focussing effect to prevent random wandering of the particles.

This problem is overcome in the ridge cyclotron by varying the magnetic field in azimuth. The average field seen by a proton in a single revolution about the center is still that required for constant frequency, but includes regions of high field intensity and "regions of low field intensity, this effect being-achieved by the provision of so-called hills (ridges) and valleys between the hills. The hills are sector-shaped ridges of iron fastened to the pole faces of the magnet, top and bottom, to produce a smaller air gap between them (for example 1.5 inches) than the gap defined by the valleys (for example 6 inches) which is the distance between the pole faces themselves. A proton orbit ina ridge cyclotron is slightly square, its curvature over the hills being. greater than over the valleys. Consequently the proton velocity has an outward radial component when it encounters 'an edge of a hill and an inward radial component when leaving the edge of a hill. Aditionally, the lines of force at the edges of the hills tend to bulge out into the valleys. The total effect of these factors is a vertical focussing at both hill edges which is sufficient at low energies, but insufficient at high energies (for example it provides only about /3 of the required focussing force at energies of 50 meV.).

The above described focussing effect is experienced Whether the edges of the hills extend radially or spirally, but the spiral arrangement makes an additional contribution to vertical focussing. The bulging of the lines of force at the hill edges now has a component in the radial direction. At one edge there will be a magnetic field decreasing radially outwardly giving a focussing effect, and at the other edge a field increasing radially outwardly, giving a defocussing effect. In each case the forces are proportional to the velocity of the particle, but the focus sing forces predominate over the defocussing forces because a particle spends more time in the focussing region than it does in the defocussing one, by reason of the shape of its orbit, and because the particle is farther from the median plane on the average during focussing than during defocussing. The net focussing effect thus obtained provides the other two thirds of the total focussing effect required at high energies such as 50 mev. i

The foregoing brief description explains the advantages of a spiral ridge type of cyclotron, all of which were known prior to the present invention. Indeed such machines have already been built and operated. However their construction introduces yet a further complication when the machine is to be variable in energy. This complication resides in the problem of maintaining the correct ma gnetic field shape as the energy i varied. Although this problem can arise in other types of cyclotron, it is particularly severe in a spiral ridge cyclotron because of the intense magnetic field developed between hills (25 kilogauss or greater in some cases) at maximum energy.

The traditional method of varying the shape of a magnetic field is by the use of trimming coil provided in addition to the main coils of the electro-magnet. In view of the strong fields involved and the comparatively wide variation desired, the trimming coils required for a variable energy spiral ridge cyclotron need to be very powerful. They are accordingly bulky and expensive, and they consume substantial quantities of power.

The object of the present invention is to provide an alternative and more economical method of exercising 'control over the shape of the magnetic field in a cyclotron or like particle accelerator, and particularly in a spiral ridge or like type of cyclotron in which the field varies in azimuth and in which this problem is especially acute. The ability to exercise such control renders the machine operable under optimum conditions at various energy levels.

To this end, the invention consists of the provision in the magnetic circuit of a cyclotron or like particle accelerator of at least one control block of a material that is ferromagnetic at ambient operating temperature and has a Curie point approximately in the range of 200 to 350 C., and means for heating said block to it Curie point. Preferably a plurality of such blocks are distributed radially in the magnetic circuit (conveniently as part of the structure of the hills of a ridge cyclotron), and provision is made for heating and cooling the blocks individually. In this way, close control can be exercised over the shape of the field and a wide variety of shape variations is made possible.

One example of a manner of carrying the invention into practice is illustrated diagrammatically in the accompanying drawings. These drawings are provided by way of example only and not by way of limitation, the scope of the invention being defined by the appended claims.

FIGURE 1 shows a spiral ridge cyclotron on a small scale, the view being 'a diagrammatic one taken on the plane from which the principal components of the machine would be seen by one of the pole faces of the electromagnet;

FIGURE 2 is an enlarged view of one of the spiral hills of the cyclotron in FIGURE 1 modified in accordance with the present invention;

FIGURE 3 is a section of the line III-III in FIG- URE 2;

FIGURE 4 is a graph illustrating magnetic field conditions of the parts shown in FIGURES 2 and 3;

FIGURE 5 is another view of a portion of the cyclotron of FIGURE 2 illustrating further details of the construction;

FIGURE 6 is a section on the line VIVI in FIG- 'URE 5;

FIGURE 7 is a further enlarged view of an element of the apparatus seen in the other views; and

FIGURE 8 is a further graph.

Referring to FIGURE 1 which shows the general arrangement of a spiral ridge cyclotron, there will be seen to be provided four hills 10 and a pair of Us 11 arranged in a vacuum chamber 12. An approximate orbit of a charged particle is shown by the dotted line 13 to illustrate the slightly square nature of this orbit by virtue of its smaller radius of curvature when in the neighborhood of the hills 10. A true particle orbit will of course be spiral in shape, but the increase in radius per orbit is too small to be visible in FIGURE 1. The valleys be- .4 tween the hills 10 are defined by the pole faces of the electromagnet (not shown) which is conventional in form and generates a field perpendicular (except for the bulging previously mentioned) to the plane on which the view of FIGURE 1 is taken.

The improvement in such machines with which the present invention is concerned is illustrated in FIGURES 2, 3 and 5 to 7, which shown a hill 10 secured to the pole face 14 of the electro magnet. In addition to having the characteristic shape of a spiral ridge cyclotron, namely with spiral edges 15 and 16, the hill 10 is made up of a flat, iron, face portion 17, an iron spacer 18, insert blocks 19 and face blocks 19'. The blocks 19 and 19 are made of Invar, which is a readily available nickel-iron alloy (36% nickel) having no significant temperature hysteresis, a saturation induction of the order of 13 kilogauss at room temperature, and a Curie point of 280 C.

FIGURES 5, 6 and 7 demonstrate in further detail the mechanical structure of the Invar blocks 19 and 19'. Typically, a block 19 is formed in two parts 19a and 1% with cooperating grooves provided in adjacent surfaces to receive a cooling coil 25, an electric heating element 26 and a temperature-sensing thermocouple 27. FIGURE 5 shows the manner in which electrical leads and cooling water connections extend to the insert blocks 19 positioned beneath the pole piece 17 and the similar connection of heating, cooling and sensing elements to a typical pair of face blocks 19'. Cover plates 19c and 19d of blocks 19' may be made of a different material such as copper soas to provide three steps 30, 31 and 32 in the Invar material. As shown in the detailed view of FIGURE 6 the blocks 19, 19 are supported out of contact with one another and with the steel of the pole pieces 14 and 17 by thin screws 33 and stainless steel standotfs 34. This arrangement minimizes heat transfer.

Close control can be exercised over the temperature of each individual Invar block and the field can thus be made to have a wide variety of shapes to suit any one of numerous possible operating conditions.

FIGURE 8 shows a typical variation in the magnetic field between the poles of a magnet with an Invar block interposed, as the temperature of the Invar block is increased. A smooth change of field intensity is obtained with increasing temperature and it will thus be apparent that close control over the field in the cyclotron can be achieved, not only spatially (by virtue of which blocks are chosen to be heated), but also in degree (by virtue of the relative extent to which different blocks are heated).

At ambient operating temperature, the field strength along the hill 10 is typically that shown in FIGURE 4 by the line 20. This is the required field shape calculated for maximum energy (for example 50 mev. protons at approximately a 20 inch radius). If now the current in the coils of the electromagnet is decreased for low energy operation, the field will tend to assume the shape shown by the line 21 in FIGURE 4. This is a field which increases much too rapidly with radius for optimum operating conditions. However, by heating the Invar blocks 19 and 19' selectively to elevated temperatures, they can be made to have little or no influence on the field, since above the Curie point the normally ferromagnetic material exhibits the characteristics of a paramagnetic material. Almost any desired field shape can be achieved, for example, the field represented by the line 22 which is the optimum shape for minim-um energy operation.

The invention is not limited to the use of Invar as the material from which the blocks 19, 19' are formed, although this is the preferred material by virtue of its excellent properties for the purpose and its ready availability. Nickel-iron alloys in the gamma phase, that is above 30% nickel have Curie points varying from approximately C. at 30% nickel to approximately 625 C. at about 65% nickel. Above this percentage the Curie point declines again. As stated above, Invar which is 36% nickel has a Curie point of about 280 C. which is ideal for the present purposes. Clearly, it is undesirable for the magnetic properties of the control blocks to be appreciably affected by normal operating temperature rise. On the other hand, the temperature rise required to enable eifective control to be exercised should not be so great as to involve heating the apparatus unduly. This consideration limits the invention to the use of materials with Curie points not much in excess of 300 C. The Curie point of the iron itself is approximately 770 C., which renders it virtually unaffected by the temperature rise of the adjoining blocks of Invar or other material with a comparatively low Curie point. Although there are no critical limits, practical considerations will normally dictate the use of a material for the blocks having a Curie point approximately in the range 200 to 350 C. The particular alloy employed will depend on availability and other desirable properties of the alloy, such as the absence or near absence of temperature hysteresis and a comparatively high saturation induction. It is important to have a material with high saturation induction, at room temperature, since the total amount of magnetic field control in going from room temperature to the Curie point is proportional to this quantity. The choice will normally be from the nickel-iron alloys or the nickel-copper alloys which can readily be obtained with a Curie point in the region of 200 C. to 350 C. The nickel-iron alloys are preferred.

The control blocks can be kept within the vacuum system of the machine so that the heat losses will be restricted to radiation and conduction through the supports. The total power required to hold the control blocks at an equilibrium temperature is only a few kilowatts, since heating is effected by the elements embedded in the blocks. The water tubes enable quick cooling. It is possible to hold the temperatures of the control blocks at any point constant in time to within approximately 0.5 C., which will keep the magnetic field at any given radius constant to approximately 1 gauss.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In the magnetic circuit of a cyclotron or like particle accelerator, a plurality of control blocks disposed as a radially extending row, each of said blocks being of a material that is ferromagnetic at ambient operating temperature and has a Curie point approximately in the range 200 to 350 C., and separate means for heating each of said blocks individually to its Curie point.

2. The structure of claim 1, wherein said material has substantially no temperature hysteresis and has a high saturation induction.

3. The structure of claim 2, wherein the material is Invar.

4. In a spiral ridge cyclotron or like particle accelerator, a magnetic circuit including spirally extending hills, a plurality of individual control blocks forming part of each of said hills and extending as a row therealong, each of said control blocks being of a material that is ferromagnetic at ambient operating temperature and has a Curie point approximately in the range 200 to 350 C, and separate means for heating each of said blocks individually to its Curie point.

5. The structure of claim 4, including means for cooling said blocks and means for sensing the temperature thereof.

6. The structure of claim 4, including additional blocks of said material at the radially outward end of each hill.

7. The structure of claim 4, wherein said material has substantially no temperature hysteresis and has a high saturation induction.

8. The structure of claim 7, wherein the material is Invar.

References Cited by the Examiner UNITED STATES PATENTS 1,901,708 3/1933 Ellingson. 2,718,569 9/1955 Johnston 3l7133 3,175,131 3/1965 Burleigh et al 3l3-62 BERNARD A. GILHEANY, Primary Examiner. G. HARRIS, Assistant Examiner. 

1. IN THE MAGNETIC CIRCUIT OF A CYCLOTRON OR LIKE PARTICLE ACCELERATOR, A PLURALITY OF CONTROL BLOCKS DISPOSED AS A RADIALLY EXTENDING ROW, EACH OF SAID BLOCKS BEING OF A MATERIAL THAT IS FERROMAGNETIC AT AMBIENT OPERATING TEMPERATURE AND HAS A CURIE POINT APPROXIMATELY IN THE RANGE 200* TO 350*C., AND SEPARATE MEANS FOR HEATING EACH OF SAID BLOCKS INDIVIDUALLY TO ITS CURIE POINT. 